THE USES OF PHOTOELECTRIC CELLS 435 



SECOND EVENING DISCOURSE 

 Tuesday, September 6, 1932. 



THE USES OF PHOTOELECTRIC CELLS 



BY 



C. C. PATERSON, O.B.E. 



The photoelectric cell has been known to science for years. It is a device 

 which passes an electric current only when light falls on it, and has been 

 evolved as a consequence of the study by physicists of the connections 

 between light, electricity and matter. The electric currents given by these 

 cells are so minute that until recently only the physicist with his delicate 

 apparatus was able to measure them, but with the advent of the thermionic 

 valve with its power of magnifying currents, the engineer-physicist realised 

 the possibility of applying the photo-cell to numerous everyday uses. 



So numerous and varied, indeed, are these applications that I can only 

 describe a few typical ones, chosen to illustrate particular properties of these 

 cells. 



The emission of negative electricity from a brightly illuminated zinc 

 plate was demonstrated by Hallwachs after he had received a hint from 

 the work of Hertz. This emission is now called the photoelectric effect. 

 {Demonstration — The image of an electroscope was thrown on to the screen, and 

 the collapse of the leaves showed the discharge of electricity from an electrically 

 connected zinc plate when this was illuminated with light from an arc lamp.) 



Modern photo-cells employ more highly sensitive surfaces than zinc, 

 and the sensitive surface is enclosed in an evacuated bulb. 



This type of cell passes an electric current strictly proportional to the 

 illumination falling on it — a property which enables the cell to be used 

 for a number of quantitative purposes, such as the measurement of area. 

 {Demonstration — A photo-cell was exposed to the light from a uniformly 

 illuminated opal window. The decrease in electric current consequent upon the 

 reduction of the radiating area by the interposition of an irregularly shaped 

 surface (e.g. an animal hide) gave a measure of the area of the surface.) 



Certain types of cell respond most strongly to light of one colour, while 

 other types are more affected by other colours. This can be demonstrated 

 by exposing a photo-cell to a spectrum thrown on a screen. {Demonstra- 

 tion — A casium cell and a potassium cell exposed to spectrcil colours gave 

 their maximum response in the infra-red and in the blue respectively.) 



We now come to consider in more detail the practical application of the 

 photo-cell. As an example of simple ' trigger-action ' we may take the case 

 of the photoelectric street-lighting unit which switches on the lights at the 

 approach of darkness, extinguishing them again in the morning. {Demon- 

 stration.) This ' trigger-action ' is also used for counting objects as they 

 pass along a moving band. {A demonstration of a photoelectric counter which 

 counted red balls rolling along a track but allowed blue balls to pass without 

 recording their passage was given ^ 



The fundamental properties of photo-cells make it possible to employ 

 them in photometry instead of the human eye. The fact that cells respond 



